feat(api): library sources, scanner and catalog (M2)

- schema for sources, path patterns, media files, artists, albums,
  tracks, genres (migration 00003)
- path patterns (RE2 with named groups, skip), merge with tags by
  source priority, fallbacks from the file name and folder, name
  normalization and multi-artist split
- scanner: walk, skip unchanged files by size and mtime, ffprobe in 8
  workers with a single catalog writer, renames by content hash keep
  the track, duplicates fold into one track with the best file as
  primary, missing files are kept 30 days
- storage-agent mounts NFS with validated server, export and an option
  whitelist, no shell; test mount counts audio files
- admin API: sources CRUD, connection test, patterns with preview,
  scan, library status, file errors, host folder picker; scheduled
  scans and health checks; mounts are restored on start
- catalog API: artist, artist albums, album with tracks, track
- api image is alpine with ffmpeg for ffprobe

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/api/internal/library/meta/merge.go

@@ -0,0 +1,137 @@
+package meta
+
+import (
+	"path"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+// UnknownArtist — исполнитель, если его нет ни в тегах, ни в пути.
+const UnknownArtist = "Unknown artist"
+
+// Fields — значения метаданных из одного источника (теги файла или шаблон пути). Пусто — нет значения.
+type Fields struct {
+	Artist      string
+	AlbumArtist string
+	Album       string
+	Title       string
+	Genre       string
+	Year        string
+	Track       string
+	Disc        string
+}
+
+// FromPath собирает Fields из групп шаблона.
+func FromPath(m map[string]string) Fields {
+	return Fields{
+		Artist: m["artist"], AlbumArtist: m["albumartist"], Album: m["album"], Title: m["title"],
+		Genre: m["genre"], Year: m["year"], Track: m["track"], Disc: m["disc"],
+	}
+}
+
+// Meta — итоговые метаданные файла.
+type Meta struct {
+	Artist      string // поле исполнителя целиком (разбивается SplitArtists)
+	AlbumArtist string
+	Album       string
+	Title       string
+	Genres      []string
+	Year        int
+	Track       int
+	Disc        int
+}
+
+// Merge сводит теги и поля из пути. tagsFirst: тег важнее пути (по умолчанию), иначе наоборот.
+// Без обоих значений: title — имя файла без номера и расширения, artist — UnknownArtist,
+// album — имя папки, albumartist — artist.
+func Merge(tags, fromPath Fields, tagsFirst bool, relPath string) Meta {
+	first, second := tags, fromPath
+	if !tagsFirst {
+		first, second = fromPath, tags
+	}
+	pick := func(a, b string) string {
+		if v := Clean(a); v != "" {
+			return v
+		}
+		return Clean(b)
+	}
+
+	m := Meta{
+		Artist:      pick(first.Artist, second.Artist),
+		AlbumArtist: pick(first.AlbumArtist, second.AlbumArtist),
+		Album:       pick(first.Album, second.Album),
+		Title:       pick(first.Title, second.Title),
+		Year:        parseYear(pick(first.Year, second.Year)),
+		Track:       parseNumber(pick(first.Track, second.Track)),
+		Disc:        parseNumber(pick(first.Disc, second.Disc)),
+	}
+	if g := pick(first.Genre, second.Genre); g != "" {
+		m.Genres = splitGenres(g)
+	}
+
+	if m.Title == "" {
+		m.Title = titleFromFile(relPath)
+	}
+	if m.Artist == "" {
+		m.Artist = m.AlbumArtist
+	}
+	if m.Artist == "" {
+		m.Artist = UnknownArtist
+	}
+	if m.AlbumArtist == "" {
+		m.AlbumArtist = m.Artist
+	}
+	if m.Album == "" {
+		if dir := path.Base(path.Dir(relPath)); dir != "." && dir != "/" {
+			m.Album = Clean(dir)
+		}
+	}
+	return m
+}
+
+var yearRe = regexp.MustCompile(`\b(1[89]\d\d|20\d\d)\b`)
+
+// parseYear достаёт год из «2019», «2019-05-01», «(1999)».
+func parseYear(s string) int {
+	if y := yearRe.FindString(s); y != "" {
+		n, _ := strconv.Atoi(y)
+		return n
+	}
+	return 0
+}
+
+// parseNumber разбирает «3», «03», «3/12».
+func parseNumber(s string) int {
+	s, _, _ = strings.Cut(s, "/")
+	n, err := strconv.Atoi(strings.TrimSpace(s))
+	if err != nil || n < 0 || n > 9999 {
+		return 0
+	}
+	return n
+}
+
+func splitGenres(s string) []string {
+	var out []string
+	seen := map[string]bool{}
+	for _, g := range splitAny(s, []string{";", "/", ","}) {
+		g = Clean(g)
+		if k := Key(g); k != "" && !seen[k] {
+			seen[k] = true
+			out = append(out, g)
+		}
+	}
+	return out
+}
+
+var leadingNumber = regexp.MustCompile(`^\s*(?:\d{1,2}[-.])?\d{1,3}\s*[-._)]*\s*`)
+
+// titleFromFile: «03 - Song name.flac» → «Song name».
+func titleFromFile(relPath string) string {
+	name := path.Base(relPath)
+	name = strings.TrimSuffix(name, path.Ext(name))
+	if t := Clean(leadingNumber.ReplaceAllString(name, "")); t != "" {
+		return t
+	}
+	return Clean(name)
+}